Early detection of the worsening of heart failure to prevent hospitalization is an ongoing challenge. Intra-thoracic impedance measurements can give a good indication of the fluid status of patients, with decreases in impedance being indicative of increases in fluid content and increases in impedance being indicative of decreases in fluid content. Knowledge of a patient's long-term impedance measurements and changes therein are a valuable clinical indicator of a patient's health.
The accumulation of fluid can indicate decompensation, failing heart circulation as well as several other conditions. There are several mechanisms or diseases that can cause or affect the accumulation of fluid. In general, fluid accumulation is a failure or over-response of the homeostatic process within the body. The body normally prevents the build up of fluids by maintaining adequate pressures and concentrations of salt and proteins and by actively removing excess fluid. Fluid accumulation can occur, for example, when the body's mechanisms for preventing fluid accumulation are affected by disease, such as heart failure, left-sided myocardial infarction, high blood pressure, altitude sickness, emphysema (all of which affect pressures), cancers that affect the lymphatic system, renal diseases, and diseases that disrupt the protein concentrations. Likewise, abnormally low fluid levels can also be problematic. As a result, providing an adequate monitor of the patient's fluid status can provide physicians and patients with a better tool to manage disease.
Determining the impedance values at which to notify a patient can prove difficult. On one hand, patients should be notified every time they are approaching a dangerously high or low fluid status. On the other hand, notifying patients when they face no dangerously high or low fluid status can result in the unnecessary consumption of time and resources. This can be especially problematic when the unnecessary consumption is multiplied over a large patient population.